The theme that I would say is most relevant is the idea that things are not always what they seem, second, reality is perception. M. Lantin believes that the jewels are fake, his wife never tells him that they are real, she can't he did not buy them for her, and she could not afford to buy them herself.
He believes that she loves him and is a faithful wife. When in fact, she must have had an affair with a wealthy man during their marriage, the person who bought the gems.
The turning point in the story, or the climax comes when M. Lantin discovers that the gems are real. His life is dramatically changed once he sells the jewels. He becomes very wealthy.
The irony in this story is verbal irony, because M. Lantin's wife knows that the jewels are real. She tells him:
"What can I do? I am so fond of jewelry. It is my only weakness. We cannot change our natures." Then she would roll the pearl necklaces around her fingers, and hold up the bright gems for her husband's admiration, gently coaxing him:"Look! are they not lovely? One would swear they were real." (de Maupassant)
The other form of irony is situational irony. When the outcome is different than expected. M. Lantin believes that the gems are false, and then discovers to his surprise that they are real.
When he marries his second wife, he expects that she will make him happy because of her virtue, the opposite happens, he is miserable.